Electric vehicle braking resistor gets a water-cooled boost
24Mar
posted by: Cressall
A braking resistor, used in electric vehicles, is also suitable for industrial applications, explains Martin Nicholls, Cressall.
The two disciplines share water cooling as means to increase power density.
There would seem to be a world of difference between the current generation of advanced electric sports cars and the heavy industries where Cressall Resistors traditionally operates. The common denominator is a lightweight 25kW water-cooled resistor.
In the middle of the last decade, electric sports vehicles, with acceleration comparable to a normal internal combustion engine sports car, began production. There are now a number of advanced models on the market. Admittedly, they do, of course, come with the need for a similarly sophisticated wallet.
Electric vehicles
The company’s involvement in the electric vehicle market allowed it to identify the need for the first of a series of EV (electric vehicle) resistors, culminating in a product the company christened the EV2. Electric cars have always been at a disadvantage against their petrol-driven counterparts, in terms of energy density. A can of petrol weighing 4kg equates to an electric vehicle battery weighing half-a-tonne, and costs €6.00 compared to €40,000.
This equation does not take into account the cost of charging, which is not too dissimilar to the cost of petrol. This is because when a user buys electricity, the cost is effectively three times as much as for the raw material, due to the inefficiency of power generation.
High capital cost and weight are exacerbated by the issue of battery life. As a result, many all-electric sports cars, such as the Tesla, are bought as status symbols and not because they provide an economic way of getting to work. Therefore the whole idea of a hybrid car is that it runs from a much smaller engine, of say 10kW, which is appropriate for the power consumed in an average car journey and which recharges the battery.
Advanced ceramic
In the meantime, the company has identified an industrial need for water-cooled resistors with minimal footprints in industrial applications. It has launched the EV2 water-cooled resistor, which is suitable for medium voltage industrial applications. This resistor features two plates made from an advanced ceramic with what the company describes as “exceptional properties”.
An important benefit of using an industrial electric drive is that reliable systems of regenerative and dynamic braking are available to complement or replace traditional mechanical braking systems. The advantages of electric braking include control, reliability, mechanical simplicity, weight saving and, in certain cases, the opportunity to make use of the regenerated braking energy to top up batteries.
On the one hand, the EV2 has the same insulation properties as any other ceramic, with insulation withstand to 20kV. However, the thermal properties are such that it will conduct heat almost as effectively as aluminium - normally a good insulator is a bad conductor.
Resistor construction
Originally developed for the electronics on the Japanese Shinkansen bullet train, the resistor allows power electronic components to be mounted on one side and a copper heatsink on the other, so it acts not only as an electrical insulator but also as a thermal conductor.
The EV2 was developed using a patented construction to manage temperature. This design was prepared using FEA (finite element analysis) software tools, allowing a peak temperature of about 350°C from an electrical load of over 25kW; although 40kW has been attained.
Launch customers for the EV2 include Rolls Royce in Canada and Korea’s Samsung for a vehicle prototype. These design projects are driven by the need for minimising onboard space previously occupied by traditional water cooled resistor designs. As the resistor is completely encapsulated, it has a lower explosion risk for offshore rigs and can conduct heat away more quickly from areas where this would be a problem, like the bowels of a ship, for example.
The search for a material with higher resistivity than stainless steel led the company to consider the use of graphite, such as that traditionally used in vacuum furnace elements. Graphite is a very stable material and has desirable ohmic values but, being pure carbon, it oxidises. The company has now found a way of encapsulating a graphite element within the EV2 to protect it from the air.
A French distributor has a customer that makes winches for offshore oil platforms which use Caterpillar drives to lower pipes down to the sea bed, and this requires braking. Using the water-cooled resistors, available at the same price as the traditional air-cooled resistors, one tenth of the space has been saved. The company plans to get certifications from both Lloyds and Atex for the resistor.
Independent braking
In 15 to 20 years, we will probably all be driving hybrid or EVs and all vehicles must have at least two or three independent braking systems for safety reasons. Typically, in a conventional car, these are dual-circuit hydraulic brakes plus a handbrake.
Wherever possible, braking will be regenerative rather than mechanical. This creates the possibility of storing and re-using the regenerated braking energy, rather than just dissipating it as waste heat. Storage of the recovered energy can be in the vehicle’s batteries or in ancillary media such as flywheels or ultracapacitors.
The EV2 water-cooled resistor is a 25kW unit, available as a single unit or as a block of 10 with a common cable box attached to 250kW braking power input, all using a common water supply. Cooling is achieved by pumping cold water, which comes into one end of the system and then absorbs the heat. It can be pumped put through a radiator, which can be located some way from the heat generating equipment.
Perhaps the needs of the automation industry and the electric vehicle sector are not worlds apart after all?
Figure 1; The EV2 braking resistor can be used in electric vehicles and industrial applications.
Figure 2; The EV2 water-cooled resistor is a 25kW unit, available as a single unit or as a block of 10.
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